Prepared by:Kansagara Ravi J. (120230109028)

AC Servomotor

Guided by : Prof.J.I.Patel sir

What is Servomotor?A servomotor is a rotary actuator that allows for

precise control of angular position, velocity and acceleration. It consists of a suitable motor coupled to a sensor for position feedback. It also requires a relatively sophisticated controller, often a dedicated module designed specifically for use with servomotors.

Servomotors are not a specific class of motor although the term servomotor is often used to refer to a motor suitable for use in a closed-loop control system.

Introduction

Servo System CompatibilityAn industrial servomotor is almost always part of a larger servo system. It is important to ensure the motor is compatible with the associated system before making a selection. This includes compatibility with the controller, amplifier, and programmable logic controller (PLC) components of the system. In operation, the PLC sends position commands specified by the user to a servo controller. These commands are interpreted by the position controller and sent to an amplifier which processes them for use and response by the motor. As the motor carries out these commands, the encoder sends position signals back to the controller, which processes them for the next set of commands.

Servo system

Speed Torque characteristics 1. The rotor of the servomotor is built with high resistance, so that its X/R (Inductive reactance / Resistance) ratio is small which results in linear speed torque characteristics.(But conventional induction motors will have high value of X/R which results in high efficiency and non-linear speed-torque characteristics). The Speed-torque characteristics of normal induction motor (Curve-a) and AC servomotor (Curve-b) are shown in figure.

2. The excitation voltage applied of two stator windings should have a phase difference of 90°.

Speed Torque characteristics of a) Normal induction motorb) AC Servomotor

induction motor and AC Servomotor

Construction of stator

The AC servomotor is basically a two phase induction motor with some special design features. The stator consists of two pole-pairs (A-B and C-D) mounted on the inner periphery of the stator, such that their axes are at an angle of 90° in space. Each pole-pair carries a winding. One winding is called reference winding and the other is called a control winding. The exciting current in the winding should have a phase displacement of 90°. The supply used to drive the motor is single phase and so a phase advancing capacitor is connected to one of the phase to produce a phase difference of 90°. The stator constructional features of AC servo motor are shown in figure.

Construction of rotor The rotor construction is usually squirrel cage or drag-cup type. Rotor Construction of AC Servo motor is shown in figure The squirrel cage rotor is made of laminations. The rotor bars are placed on the slots and short circuited at both ends by end rings. The diameter of the rotor is kept small in order to reduce inertia and to obtain good accelerating characteristics.

The Drag-cup construction is employed for very low inertia applications. In this type of construction the rotor will be in the form of hollow cylinder made of aluminium. The aluminium cylinder itself acts as short circuited rotor conductors. (Electrically both the types of rotor are identical).

Working of Servomotor as ordinary induction motor

The stator winding are excited by voltages of equal rms magnitude and 90° phase difference. These results in exciting currents i1 and i2 that are phase displayed by 90° and have equal rms values. These current give rises to a rotating magnetic field of constant magnitude. The direction of rotation depends on the phase relationship of the two currents (or voltages). The exciting current produces a clockwise rotating magnetic field and phase shift of 180° in i1 will produce an anticlockwise rotating magnetic field.

Working Principles Of AC Servomotor

The rotating magnetic field sweeps over the rotor conductors. The rotor conductors experience a change in flux and so voltages are induced in rotor conductors. This voltage circulates currents in the short circuited rotor conductors and the currents create rotor flux. Due to the interaction of stator and rotor flux, a mechanical force (or torque) is developed on the rotor and so the rotor starts moving in the same direction as that of rotating magnetic field.

The symbolic representation of an A.C servomotor with control system component is shown in figure The reference winding of A.C servomotor is excited by a constant voltage source with frequency of 50Hz. The speed of A.C servomotor is controlled by controlling the control voltage. The error output of error detector is fed to PI controller, due to the error, controller take control action (i.e. to give control voltage) to firing circuit. The firing circuit generates the pulse's to rotate the motor at required speed.

Working Principles of AC Servomotor in control system

Symbolic representation of AC Servomotor in control system.

Characteristics Curves of AC Servomotor The speed Torque characteristics of a servomotor is very important for its performance in a servo system. The shape of these characteristics largely depends on the value of the rotor resistance. The curves showed in figure show typical speed torque characteristics for several values of rotor resistances.Speed Torque characteristics of Servomotor with several values of resistance

It can be seen that for higher values of rotor resistance the torque speed characteristics is linear. If the rotor resistance is either R4 or R5, the motor is referred to as two phase servomotor. For servo application the motor characteristics should be linear with negative slope (With positive damping). It can be proved that for stable operation positive damping is essential. Therefore ordinary two phase induction motor with low rotor resistance is not suitable for servo applications.

It can be seen that for higher values of rotor resistance the torque speed characteristics is linear. If the rotor resistance is either R4 or R5, the motor is referred to as two phase servomotor. For servo application the motor characteristics should be linear with negative slope (With positive damping). It can be proved that for stable operation positive damping is essential. Therefore ordinary two phase induction motor with low rotor resistance is not suitable for servo applications.

SERVO MOTOR DRIVE

Outer look of AC servo motor

Nameplate of AC servomotor

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